Many modulation standards use what is referred to as “non-coherent modulation”. As shown in FIG. 1, non-coherently modulated signal 105 is formed by modulating carrier signal 110 with modulating signal 115 produced by symbol generator 120. Symbol generator 120 generates modulating signal 115 from symbol clock 125. Symbol clock 125 and carrier signal 110 are asynchronous, that is, they are not phase-locked in any way. Modulator 130 may modulate carrier signal's 110 amplitude, phase, or both depending on the particular modulation standard used.
Non-coherently modulated signals are typically measured using a real-time oscilloscope or a real-time spectrum analyzer. However, in some cases the bandwidth of these signals exceeds what is available from commercially available instruments. For example, some signals used in satellite communications have a carrier frequency of 60 GHz and multi-GHz of bandwidth. Such a signal is too high in frequency for a commercially available real-time oscilloscope, and too wideband for a commercially available real-time spectrum analyzer. An equivalent-time sampling oscilloscope has enough bandwidth to measure such a signal, but unfortunately a sampling scope requires that a signal under test be repetitive in order to produce a stationary single-valued waveform, and a non-coherently modulated signal is not repetitive. That is, even if the modulating signal is repetitive, the modulated carrier will not be repetitive because the symbol clock and the carrier signal are asynchronous.
A need remains for a way to address these and other problems associated with the prior art.